1. Field of the Invention
This invention relates to a method for synthesizing high density liquid hydrocarbon fuels.
2. Description of the Prior Art
High density liquid hydrocarbon fuels are characterized in having a net volumetric heat of combustion in excess of about 140,000 BTU per gallon. A high density or energy fuel is essentially required for fueling turbojet and ramjet propelled limited volume missile systems. Beyond the need for a high energy content in order to achieve an adequate range performance of the missile, there are other requirements in the forefront depending in the main on the manner in which the missile is to be deployed. For instance, the operational conditions encountered in the deployment of certain missile systems call for the use of a fuel having the highest energy content consistent with having a freeze point substantially below 0.degree. F. and being reasonably fluid at a temperature near its freeze point.
A high density fuel of the foregoing type does not occur in nature but rather must be chemically synthesized. Essentially all of the current generation of such fuels commonly feature a norbornane moeity having an additional cyclic hydrocarbon appendage. A particularly noteworthy fuel of the foregoing type is represented by the exo stereo isomer of tetrahydrodicyclopentadiene which in commerce is referred to as JP-10 fuel. The latter is prepared by first hydrogenating dicyclopentadiene yielding the solid endo-isomer of the hydrogenated derivative. The endo structure is then isomerized in the presence of a catalyst to produce the exo-isomer almost quantitatively in a relatively pure form. Since JP-10 is derived from an abundantly available raw material coupled with the fact that the indicated isomerization procedure is highly effective and relatively facile, such are the main factors why the product is regarded to be an important high energy fuel.
Since JP-10 has a heat value of about 140,000 BTU per gallon it is not used by itself to fuel the more advanced second generation turbojet missile systems where maximum range performance is of primary consideration. In order to obtain a composite fuel of a suitable heat value for the indicated purpose, JP-10 is blended with the highest energy fuel currently available; viz., RJ-5, a structurally related fuel having a heat value of 161,000 BTU per gallon. This fuel is chemically described as the hydrogenated dimers of norbornadiene. The process for producing RJ-5 initially involves dimerizing norbornadiene in the presence of a zero valent iron complex. The dimerization step in accordance with the prior art results in the formation of three principle stereo isomer species with the major species being the endo, endo isomer, generally referred to simply as the unsaturated HNN isomer. Due to its substantial HNN content, RJ-5 exhibits such a high freezing temperature that it can be blended with JP-10 only to a limited extent before creating fuel delivery problems of significant magnitude in the missile systems referred to above.
Accordingly the object of this invention is that of beneficially modifying the isomeric makeup of the unsaturated dimer precursors of RJ-5, specifically the HNN content thereof, which upon hydrogenation provides a liquid eutectic mixture capable of blending with a minor proportion of JP-10 to produce a fuel of improved low temperature performance capability.